A. Technical Field
The present invention relates to a process for classifying a particulate hydrophilic polymer and to a sieving device. More particularly, the invention relates to a process for classifying a particulate hydrophilic polymer in particle size with high accuracy and productivity, and further to a sieving device suitable for such a classification. Examples of the particulate hydrophilic polymer include: water-soluble polymers as favorably used for materials such as flocculants, coagulants, soil improvers, soil stabilizers, and thickeners; and water-absorbent resins which are applied to wide uses, for example, as absorbing agents for sanitary materials (e.g. sanitary napkins and disposable diapers), or as water-holding agents and dehydrators in the agricultural and gardening field and the field of civil engineering works.
B. Background Art
Dry classification such as air classification and sieving are generally employed in classification operations of powdery or granular materials. It is said that the air classification is suited for classifying powdery or granular matters which are so fine that the particle diameter thereof is, for example, not more than 300 .mu.m. However, the air classification has problems in that it requires a large device. In contrast, a device as needed for the sieving is smaller than that as needed for the air classification. However, the sieving has problems in that its classification efficiency is low or its classification ability is inferior for classifying powdery or granular matters which are so fine that the particle diameter thereof is, for example, not more than 300 .mu.m.
Particularly, when particulate hydrophilic polymers are classified by conventional processes, a screen mesh face might be clogged in a short period of operation to deteriorate its classification efficiency and classification ability. In addition, there are problems in that where the separation particle diameter is so small as is not greater than 300 .mu.m, particles of large particle diameter mingle into the resultant product comprising particles of small particle diameter as have passed through a screen mesh face. Especially, sieving devices in which screen mesh faces are driven spirally, e.g., Tumbler-Screening machines as were recently developed by Allgaier Inc., exhibit high classification ability and are available for classifying fine particles. However, as the classification ability of such sieving devices becomes higher, the above-mentioned problems are greater, and it becomes more impossible to make the sieving devices exhibit their inherent high classification ability.